1. Field of the Invention
This invention generally relates to a two-axis or two-dimensional table assembly, and, in particular, to an X-Y drive apparatus of such an X-Y table assembly suitable for use in positioning a desired object, such as an electronic component, at a desired location, such as a mounting position on a printed circuit board.
2. Description of the Prior Art
An X-Y table assembly and an X-Y drive apparatus for use in such an assembly are well known and widely used in various applications. A typical prior art X-Y drive apparatus is described and claimed in the U.S. patent application No. 07/799,493 filed Nov. 27, 1991, claiming the Convention Priority from the Japanese Utility Model Application No. 2-126630, both of which have been assigned to the assignee of this application and are hereby incorporated by reference. However, the X-Y drive apparatus described in this earlier patent application uses a ball screw shaft and a worm and gear combination for transmitting power between X and Y table units.
Another typical prior art X-Y drive apparatus is schematically illustrated in FIG. 4 of this application. It is to be noted that although FIG. 4 illustrates in exploded view the detailed structure of one of X and Y table units of an X-Y drive apparatus, the illustrated structure may be incorporated into an X-Y drive apparatus shown in FIG. 1.
As shown in FIG. 1, an X-Y drive apparatus typically includes a pair of X and Y drive units 1 and 2 which extend in X and Y directions, respectively, on a common plane. Since the X and Y table units 1 and 2 are identical in structure, only the detailed structure of the X table unit 1 is shown in exploded view in detail in FIG. 4.
As shown in FIG. 1 , the X-Y drive apparatus includes the X table unit 1 which is typically fixedly mounted on an object, such as a stationary frame, which is elongated in the X direction and includes an X table 3 which is supported to move in the X direction back and forth. The X table 3 is provided with a pair of side projections 3a which extend upwardly from the opposite sides of the table 3, and the Y table unit 2 is fixedly mounted on the side projections 3a of the X table 1 so that the Y table 2 may move in the X direction back and forth as supported by the X table 3. The Y table unit 2 is elongated in the Y direction, which is perpendicular to the X direction in the illustrated example, and the Y table unit 2 also includes a Y table 3 which may move in the Y direction back and forth. As a result, the Y table 3 of the Y table unit 2 may move in any direction on a plane which is parallel to a surface on which the X table unit 1 is fixedly mounted.
As shown in FIG. 1, each of the X and Y table units 1 and 2 is generally in the form of an elongated box including an elongated bottom plate 30, a pair of elongated side plates 5, a pair of end plates and a top cover plate 10 which is somewhat narrower in width to thereby define a gap between each side of the cover plate 10 and a corresponding one of the side plates 5.
The detailed internal structure of each of the tables units 1 and 2 in a typical prior art X-Y drive apparatus is shown in FIG. 4 and a detailed description for the X table unit 1 will be had below with reference to FIG. 4. As shown in FIG. 4, the X table unit 1 includes a motor 6 which is fixedly mounted on the side plate 5 at its leftmost end. The motor 6 has a motor shaft 6a which extends in the Y direction and into the center hole of a pulley 7, so that the pulley 7 rotates in unison with the motor shaft 6a. Another pulley 8 is rotatably provided in the X table unit 1 at its other end and an endless belt 9 is provided as extending between the pulleys 7 and 8. The X table 3 is fixedly attached to the upper run of the endless belt 9 so that the X table 3 moves in unison with the movement of the endless belt 9.
In the structure shown in FIG. 4, the X table 3 is generally square or rectangle in shape and it has a width which allows the X table 3 to be fit in a space defined between the pair of side plates 5, so that the X table 3 can travel in the X direction in the space defined between the pair of side plates 5. The X table 3 is provided with a pair of side projections 3a along its opposite sides, and these side projections 3a extend through the gaps between the top cover plate 10 and the pair of side plates 5 and extend above the top cover plate 10 so that the Y table 2 may be fixedly attached to the X table 3 via its side projections 3a. The side projections 3a may be formed integrally with the rest of the X table 3 or they may be formed separately and fixedly attached to the rest of the X table 3.
Inside the X table unit 1 is also provided a pair of linear motion guide units 11 located on both sides of and in parallel with the endless belt 9 as shown in FIG. 4. Each of the pair of linear motion guide units 11 has a substantially identical structure and includes a rail 13 which is fixedly mounted on the bottom plate 30 and a slider 15 which is slidably mounted on the rail 13. Such a linear motion guide unit is well known in the art and thus its detailed description will not be repeated here. Any well known linear motion guide unit may be employed in the present invention. In the illustrated structure, the rail 13 is elongated and formed with a pair of guide grooves 12 on both sides surfaces thereof extending in parallel with the longitudinal axis of the rail 13. The slider 15 is generally saddle-shaped and thus slidably mounted on the rail 13 in a straddling manner. Although not shown specifically, the slider 15 includes a plurality of rolling members 14, such as balls or rollers, at least some of which are partly received in the corresponding groove 12 to thereby provide a rolling contact between the rail 13 and the slider 15. In one embodiment, a pair of endless circulating paths may be provided in the slider 15, in which case the rolling members 15 may roll endlessly along the endless circulating path so that the slider 15 may linearly move along the rail 13 as long as the rail 13 extends.
As shown in FIG. 4, the X table 3 is also fixedly mounted on each of the sliders 15 on both sides of the endless belt 9 and thus any load applied to the X table 3 in the downward direction is borne by either one or both of these sliders 15 or the left and right linear motion guide units 11.
In operation, when the motor 6 is energized, the pulley 7 is driven to rotate in a selected direction so that the endless belt 9 starts to travel in unison. As a result, the X table 3 also moves in unison with the endless belt 9. Depending on the direction of rotation selected for the motor 6, the X table 3 moves either to the left or to the right. The range of movement of the X table 3 is typically determined and controlled by a microcomputer (not shown) which is typically connected to a control circuit (not shown) which also controls the operation of the motor 6. In addition, although not shown specifically, a detector is typically provided inside the X table unit 1, for example, as mounted on the bottom plate 30 for detecting the position of the X table 3 along the X axis.
As indicated before, the Y table unit 2 is fixedly mounted on the X table 3 of the X table unit 1 through the side projections 3a so that the Y table unit 2 and the X table 3 move in unison at all times. As also described before, since the Y table unit 2 is identical in structure to the X table unit 1, the Y table unit 2 has a Y table 3 which may move back and forth along the Y direction which is perpendicular to the X direction. As a result, the Y table 3 may move in any direction in a plane in parallel with the surface on which the X table unit 1 is fixedly mounted. In this sense, the Y table 3 in the illustrated structure may be said to constitute an X-Y table which may move in any desired direction in its plane relative to the surface on which the X table unit 1 is mounted.
However, in the prior art structure as described above, since the endless belt 9 extends between the pair of pulleys 8 and 9, the amount of movement of the belt 9 is necessarily limited to that of the X table 3. Besides, since the Y table unit 2 is mounted on the X table unit 1, the total weight of the Y table unit 2, including its motor 6, is applied to the X table 3 of the X table unit 1 so that an increased capacity is required for the motor 6 of the X table unit 1. In addition, in the case of occurrence of a backlash or the like, since the belt 9 and the X table 3 are same in the amount of movement, an error may be produced by such a backlash.